In determining the unknown concentration of known substances by photometric measurements, two techniques are available to relate the actual light intensities received from the sample under test to the unknown concentrations of the substances being quantified in the sample under test.
One of these techniques is the "Calibration Technique". In this technique, the actual light intensities received after transmission through or reflectance from a sample having known concentrations of the substances being quantified are utilized to produce a "calibration curve". Thereafter, actual light intensity measurements obtained from a sample having unknown concentrations of the substances being quantified can then be utilized relative to the calibration curve to quantify the concentrations (S) of such substances.
The "calibration technique" has been commonly used in catheter oximetry. An optical catheter may be inserted into the blood stream of a patient and the blood oxygenation of the patient varied by having the patient breathe mixtures enriched with or depleted of oxygen. While light intensity measurements are made, blood samples are withdrawn, usually through the catheter. The oxygen saturation of these blood samples are then independently measured on a separate instrument, often in a central laboratory. After these measurements have been completed, oxygen saturation can be determined, by comparison of actual light intensities measurements relative to the calibration curve which was derived from the two known conditions of oxygen saturation. Such a calibration curve may be introduced electronically into the catheter oximeter system so that automatic computed oxygen saturation may be displayed.
This technique has several disadvantages. First, blood oxygen saturation levels are imposed upon the patient which may be deleterious to his health. Second, there is an undesirable delay between the time of catheter placement and the time at which oxygen saturation measurements utilizing the catheter oximeter can be obtained. Third, changes in blood oxygen level occur continuously and often very rapidly, making it difficult to be certain that the blood sample and the actual light intensity reading are truly correlated.
In order to eliminate the first and second disadvantages referred to above, efforts have been made to precalibrate the catheters in blood samples or suspensions of other materials such as milk of magnesia combined with dyes or filters which are believed to produce light intensity measurements equivalent to blood of known oxygen saturations. (See Taylor et al., Journal of the American Medical Association, Aug. 14, 1972, page 669; Frommer et al., The American Journal of Cardiology, May 1965, page 672; Gamble et al., Circulation, March 1965, page 331).
These calibration techniques have many disadvantages. In all of them, sterility of the catheter and of the liquid sample is difficult to maintain. In all of them, the materials in suspension (e.g. red blood cells or magnesium oxide particles) tend to be non-uniform and tend to settle. If settling is prevented by stirring the samples, the flow patterns are highly variable at different measurement sites within the sample and the flow profile and the resultant orientation of red blood cells or chemical particles is usually not similar to that found in vivo. Lastly, manipulations utilizing liquid suspensions and dyes or filters to simulate the changes produced in actually measured light intensities measured as a function of changes in blood oxygen saturation have not been satisfactory.
Another technique commonly used to relate actual light intensity measurements to the concentration of known substrates under test may be referred to as the "Differential Spectrophotometric Technique". In this technique, "Reference" light intensity measurements I.sub.o (either transmission or reflectance) are made on a material having optical properties similar to the material to be tested but lacking the specific substances to be quantified. Thereafter, actual light-intensity measurements I.sub.s may be made on the material under test including the substances to be quantified, and these light intensity measurements I.sub.s are referenced against such "Reference" light intensity measurements I.sub.o previously obtained. The substance of interest can then be quantified from the known relationship between the concentrations of the substances and the actual light intensity measurements I.sub.s normalized to the reference light intensity measurements I.sub.o.